Vehicles that have an internal combustion engine are equipped with a fuel tank to store and supply the fuel to the engine. The fuel tank is provided with a fuel level detector to detect the amount of fuel remaining therein.
In one conventional device equipped with a fuel level detector to detect the fuel level in the fuel tank, as disclosed in JP Laid-Open No. H07-91332, a fuel gauge is located to output voltage based on the fuel level. The maximum or minimum output value of the fuel gauge is memorized or learned in a backup RAM, and, based on a ratio between the learned value and the output value, relative fuel level is calculated. Also another device, as disclosed in JP Laid-Open No. H09-287997, is equipped with a fuel level detector to detect the fuel level in a fuel tank. Fuel level data is compiled into one parcel block and is stored in RAM as block data. The stored block data is read as needed and is effected by process of simple average, weighted average, or outlying value removal. This processed fuel level is indicated by a fuel meter. Further, in another air-fuel ratio controller for an internal combustion engine, as disclosed in JP Laid-Open No. H10-122016, elements resultant of the engine combustion variation are extracted from the rotational speed variation detected by a rotational speed detector, and based on these extracted elements, the air-fuel ratio of a mixture supplied to the engine is controlled to a limited lean ratio. When calculating the rotational speed variation, the moving average is filtered with primary and secondary Butterworth filters. If this result is one, the rotational speed variation is calculated from the result filtered with the primary filter. If the result is zero, the rotational speed variation is calculated from the result filtered with the secondary filter.
Conventionally, in the prior fuel level control system for the internal combustion engine, false detection of fuel level may occur due to the variation of the voltage output at start and stop of the engine that is output by the fuel level detector based on the fuel level. That is, it is mistaken for the situation of refueling even if there is no reduction/increase in the fuel level. On this account, the undesirable false diagnosis may occur while an OBD (on board diagnostics; for leak check during stop of the vehicle) is performed to determine whether the vehicle is refueled.